1. Field of the Invention
The invention relates generally to image sensors. In particular, it relates to a method of reading imaging data from the sensor.
2. Background of the Invention
There has been known an image sensor in which transfer CCD's are arranged in the row and column directions and photoelectrically converted signal charges are sequentially transferred as an output first in the column direction and then in the row direction.
In such an image sensor constituted as described above, an increase in the number of picture elements in the horizontal direction for achieving high resolution of picture elements has resulted in the phenomenon that reading of charges becomes impossible because of the limit in high frequency driving. Therefore, it has been attempted, for example, to provide a dual channel CCD and data can be read from assigned ones of the charges to be transferred horizontally.
However, this method has a disadvantage that the assignment of charges is complicated. Accordingly, a device which uses CID (charge injection device) for reading of charges in the horizontal direction has been considered in place of such dual channel CCD.
As is well known, a CID is composed of a plurality of MOS electrode pairs arranged in the form of a matrix. Image charges are proportional to the amount of incident light and are accumulated in an inversion layer under the selected electrode. Charges of only the designated electrode pairs are injected into a substrate for reading.
However, since the photosensing part and charge injection part of a conventional CID are formed in the same manner and the element surface is covered with polysilicon, its sensitivity is not so good due to the transmissivity of light. In particular, deterioration in sensitivity to the blue part of the spectrum has limited the total sensitivity of the device.
In addition, the time of about several tens to hundreds of microseconds has been required for the recoupling process in which charges are injected into a substrate. As a result, charges generated in adjacent areas enter the nearby picture element, often generating adjacent crosstalk.
Moreover, since the CID extracts the signal from the entire part of substrate on the occasion of injecting charges into the substrate, the signal-to-noise S/N ratio has been lowered due to the noise of entire part of substrate, for example, incoming noise or clock noise, etc.